I used to live in Methuen, and know that whole area and Lawrence very well. Generally, these are pretty good rental markets/ plenty of tenants available.
Of course each town has their different areas, etc. You know how that goes.
I believe that Trulia may have some of the rental data you are seeking.
Thanks and good luck.
Give this site a look for rent info and comparables..
Searching by zip code seems to work easiest/fastest
Good luck, Dunes
PS..This site might also come in handy..http://www.rentometer.com/
Like any city or town, all of these cities have areas that are more desirable areas than others. If you are considering resale value (location, location, location) you may want to stay closer to the more desirable areas. If you have any questions about the area feel free to contact me.